Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors (FET, OFET) [see H. E. Katz et al., Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition,it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
A known compound which has been shown to be an effective p-type semiconductor for OFETs is pentacene [see S. F. Nelson et al., Appl. Phys. Lett. 1998, 72, 1854] which has a planar, highly delocalised pi electron system, essential for good semiconducting properties such as charge carrier mobility. When deposited as a thin film by vacuum deposition, it was shown to have carrier mobilities in excess of 1 cm2 V−1 s−1 with very high current on/off ratios greater than 106. However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films.
One requirement for large scale, low cost manufacture is that fabrication steps be carried out by solution processing. This allows the possibility for roll-to-roll processing, where large areas can be coated and printed at high speed. Key requirements for this technique is that the material has sufficient solubility in a process appropriate solvent to ensure that the solution wets and coats the surface, and that the film formed is coherent. In addition, for a multilayer structure, it is essential that the solvent used in deposition of a layer does not effect the layer on which it is in contact with. As each layer is applied via solution, then a solvent with an incompatible solubility parameter to the previous layer is required. This solvent parameter latitude can be significantly widened if the previous layer can undergo a chemical process such as crosslinking, which effectively eliminates solubility an any subsequent processing solvent.
It has also been shown that [see Sirringhaus Appl. Phys. Lett., 77(3) (2000) 406-408] molecular self organisation into well ordered macro domains, as can be achieved in the liquid crystalline phase, also improves charge carrier mobility. Utilisation of a liquid crystal phase to both order and align molecules in a preferred orientation and direction is therefore advantageous in optimisation of semiconducting properties. In order to align and order during processing timescales and temperatures, small molecules are preferred, although polymers offer stability advantages.
It was an aim of the present invention to provide new organic materials for use as semiconductors or charge transport materials, which satisfy all criteria discussed in that they are easy to synthesize, have high charge mobility through molecular orbital delocalisation through an oxidatively stable pi conjugated system, can be oriented and aligned during processing to form a closely packed, stable morphology, and is ammeanable to a multilaminar solution process. The materials should be therefore be easily processible to form thin and large-area films for use in semiconductor-devices. Other aims of the invention are immediately evident to the skilled in the art from the following description.
The inventors have found that these aims can be achieved by providing polymerisable compounds having two or more polymerisable groups that are attached optionally via spacer groups, to a core imparting charge transport properties to the compound. These compounds are more effective than conventional organic materials (conjugated oligomers and polymers) as charge transport materials due to the following features. Firstly, the highly ordered nature of the mesogenic compounds in the mesophase (for example in the smectic phase) will result in a denser, more closely packed structure than an amorphous polymer, facilitating intermolecular charge-hopping mechanisms and so increasing charge carrier mobilities. Furthermore, the polymerisable end-groups can be crosslinked to freeze the ordering of the mesophase in the structure, and densify the structure even further. Compared to standard mesogenic materials, crosslinked materials obtained from the polymerisable compounds of the present invention have improved thermal and solvent resistance, and are thus impervious to further solution processing steps that are necessary for the fabrication of an electronic device.
A further aspect of the invention relates to liquid crystal polymers, in particular liquid crystal side chain polymers obtained from the reactive mesogens according to the present invention, which are then further processed e.g. from solution as thin layers for use in semiconductor devices.
JP2000-347432-A and JP11-209761 describe liquid crystal charge transport materials having a polymerisable group that can be polymerised to form polymer films. However, they do not disclose compounds with more than one polymerisable group.
Grell et al., J. Korean Phys. Soc. 2000, 36(6), 331 suggest a reactive mesogen comprising a conjugated distyrylbenzene core with two reactive acrylate end groups as a model compound for molecular electronics. However, there is no disclosure if the shown compound has suitable charge transport properties, or how it can be processed to form a device.
Definition of Terms
The terms ‘liquid crystalline or mesogenic material’ or ‘liquid crystalline or mesogenic compound’ means materials or compounds comprising one or more mesogenic groups, for example rod-shaped, lath-shaped or disk-shaped groups, i.e. groups with the ability to induce liquid crystal phase behaviour. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behaviour only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerised.
The term ‘polymerisable’ includes compounds or groups that are capable of participating in a polymerisation reaction, like radicalic or ionic polymerisation from unsaturated functionality, polyaddition or polycondensation, and reactive compounds or reactive groups that are capable of being grafted for example by condensation or addition to a polymer backbone in a polymeranaloguous reaction.
The term ‘film’ includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.